a. Field of the Invention
This invention relates to a method and a system for preventing pitch material in the pulp stock of paper making machines from adhering and building up on the fabric support surfaces of support elements such as foil blades and suction boxes used in such machines to remove water from the pulp in the forming section and to clean and condition the felts in the press section
B. Description of Prior Art
The usual paper making machine comprises in order, a forming section, press section and dryer section. In the forming section the water content of the stock is lowered from approximately 99.5% to the order of 80 - 85%. This is done as the stock, which has been spread on a travelling forming fabric, passes over a series of drainage foils and suction boxes. The forming fabric usually consists of a fine mesh woven metal or synthetic fabric which offers good support to the pulp stock and, at the same time, permits controlled drainage of some of the water therefrom.
After the sheet has been formed and removed from the forming section at the couch roll, a considerable percentage of the remaining 80 - 85% of water may be removed by means of pressing or squeezing the sheet in the press section before starting the more expensive water removal by means of vaporization through heat in the dryer section. The pressing or squeezing is done by passing the sheet through the nips of a series of press rolls while it is being supported on a press felt. The press felt is a relatively thick fabric of which one or both surfaces may be composed of a needled bat of synthetic or natural fibres.
In the forming section of the paper making machine the fabric runs over the forming board, foils and the suction boxes, and some of the water is drawn from the stock. Pitch material carried in the water or in the forming fabric tends to build up on the top surfaces of these components with detrimental results as their effectiveness is lowered. On the foils it may interfere with the critical leading edge and foiling angle and on the suction boxes it may eventually bridge over the holes or slots leading to the interior of the boxes and so reduce the effect of the vacuum therein. These components must then be removed for cleaning which may necessitate shutting down the whole machine. Pitch is a gummy tar-like substance which easily adheres to such components.
In the press section the press felt tends, after a time, to become loaded with debris from the sheet such as lint, chemicals and particularly pitch. Attempts have been made to clean the felt by a type of suction pipe which is placed in contact with the paper side surface of the felt. A simple version of such a suction pipe would comprise a pipe extending across the width of the felt in a run between two turning rolls. There would be a narrow slot opening into the pipe and running lengthwise of it. The pipe would be positioned so that the felt runs over the slot and the slot extends almost the entire width of the felt. A vacuum is supplied in the pipe which serves to draw water and debris off the surface of the felt. Usually a high pressure shower is impinged on the opposite side of the felt immediately preceding the suction pipe and this serves to dislodge foreign material which may then be more easily removed by the suction.
In order to improve the efficiency of this felt cleaning device it is usually fitted with a cover comprising spaced bars of material which resists wear and deterioration such as polyethylene. The slot or slots between the bars coincide with the slot leading into the interior of the pipe. Deckle pieces are provided to seal the cover slots from extending beyond the edges of the press felt. Such an arrangement is called a felt suction box or Uhle box.
Since modern forming fabrics and press felts are not worn out as quickly as conventional ones previously used, the extended life has laid more emphasis on the problem of keeping the machine and the felts clean of fibre fines, chemicals and pitch. These foreign materials adversely affect the efficiency of the foils and suction boxes of the forming section and clog the mat surface of a press felt and greatly reduce its porosity and therefore its water removing effectiveness.
In the case of the press felts, for example, this has made the efficient use of the Uhle boxes of even greater importance and particular attention must be paid to maintaining their efficiency by keeping them clean. In the case of most paper machines a build-up of pitch occurs on a Uhle box, starting at the leading edges of the bars or blades, and it becomes necessary to remove a box or individual blades periodically for cleaning off the pitch which would otherwise build up and obstruct the vacuum slot or cause damage by dropping off unto the run of the felt below.
The frequency with which cleaning off pitch is required in some operations, particularly from Uhle boxes, poses a real production problem and it is a feature of this invention to eliminate or at least alleviate the collection of pitch on foil blades, suction boxes and Uhle boxes.
Pitch which originates from resins in the wood usually occurs in the pulp stock as a gummy residue that forms in globules during the chemical treatment of wood pulp. These globules are carried in thin suspension in the pulp stock and find their way, via the forming fabric to the foils, etc. and from the surface of the formed paper sheet, to the press felt. The globules deposit on the felt or may be squeezed out of wood cells within the sheet. They accumulate and eventually interfere with the free passage of water through the felt. Some of the globules of pitch that are washed to the surface of the felt by the aforementioned high pressure shower then tend to deposit on the cover bars of the Uhle boxes as previously described. It has been found that the globules of pitch are invariably polarized.